In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
Currently mainly three solutions for holographic storage systems are discussed. In the collinear system, as disclosed for example in EP 1 624 451, separate parts of the objective lens aperture are used for the object beam and the reference beam, respectively. This arrangement is a so-called coaxial system, i.e. the object beam and the reference beam run along the same axis. This system uses a kind of shift multiplexing as a multiplexing method.
In the off-axis recording system, as disclosed for example in U.S. Pat. No. 6,721,076, the object beam and the reference beam do not share the same optical path. In this system angle and polytopic multiplexing are used.
The basic idea of the common aperture recording system, which is described, for example, in WO 2006/003077, is that the object beam and the reference beam(s) fill the full aperture of the objective lens. The common aperture system is hence a special type of coaxial system. For read-out the beams are separated in the focal plane, i.e. the Fourier plane of the reconstructed hologram image. This is different from the collinear concept, where the object beam and the reference beam only fill a distinct part of the aperture and, as a consequence, are separated in the image plane of the hologram. The common aperture system allows to achieve a higher data capacity, but the setup is more complex and instable, as the object beam and the reference beam(s) have to be separated, formed and joined.